Hither, PS plates having a constitution in which an oleophilic photosensitive resin layer is provided on a hydrophilic support have been widely used as a lithographic printing plate. With respect to the production method thereof, in general, desired printing plates have been obtained by after mask exposure (surface exposure) through a lith film, dissolving and removing a non-image area.
In recent years, digital techniques for electronic processing, storage and output of image information using a computer have become widespread, and a variety of new image output systems corresponding thereto have been put into practical use. As a result, a computer-to-plate (CTP) technique in which light having high directivity, such as laser, is scanned according to digitalized image information, thereby directly producing a printing plate without the aid of a lith film is desired, and it is an important technical problem to obtain a printing plate precursor adaptive thereto.
As one of systems of obtaining such a scanning exposable lithographic printing plate, a system of forming an ink absorbing resin layer region on a support having a hydrophilic surface is employed. This is a material in which a negative working photosensitive layer that is cured upon scanning exposure to form an ink absorbing region is provided on a support, and a constitution using a photopolymerization system composition excellent in photosensitive speed is already proposed and put into practical use. A lithographic printing plate precursor of such a constitution is simple in development processing and has desired printing performance such that it is excellent in resolution, ink acceptability, printing resistance and staining property.
The foregoing photopolymerizable composition basically contains an ethylenically unsaturated bond-containing polymerizable compound and a photopolymerization initiation system, and a binder resin, if desired. In the photopolymerizable composition, the photoinitiation system absorbs light upon scanning exposure to generate an active seed such as an active radical, thereby causing and promoting polymerization reaction of the polymerizable compound, and an exposed region is cured to form an image.
In such a scanning exposable photopolymerizable composition, a variety of photoinitiation systems having excellent photosensitivity are disclosed (for example, see Bruce M. Monroe, et al., Chemical Revue, Vol. 93 (1993), pp. 435-448, and R. S. Davidson, Journal of Photochemistry and Biology, A: Chemistry, Vol. 73 (1993), pp. 81-96). In the case where the photoinitiation system described in these documents is applied to the conventional CTP system using, as an exposure light source, a visible light source having a long wavelength such as Ar laser (488 nm) and FD-YAG laser (532 nm), in the present circumstances, a sufficient sensitivity is not obtained due to the matter that an output of the light source is not sufficiently high or other reasons. Therefore, initiation systems of high sensitivity that are adaptive to exposure at a higher speed are desired.
On the other hand, in recent years, for example, semiconductor lasers using an InGaN based material and capable of continuously causing oscillation in a region of from 350 nm to 450 nm are in the stage of practical use. Such a scanning exposure system using short wavelength light sources has such an advantage that an economical system can be constituted while keeping a sufficient output since the semiconductor laser can be produced at low costs from the structural standpoint. Further, the system can use photosensitive materials having photosensitivity in short wavelength regions and capable of working under a lighter safe light compared with existent systems of using FD-YAG or Ar laser. However, no photoinitiation systems having sufficient sensitivity against the scanning exposure in the short wavelength region of from 350 nm to 450 nm have not yet been known at present.
In view of the above, while the present applicants proposed a photosensitive composition containing highly sensitive sensitizing dyes in the short wavelength region (refer to JP-A-2000-258910), further improvement for the sensitivity of the photoinitiation system containing such sensitizing dyes has been demanded.
Further, for example, as described in J. P. Faussier, Photoinitiated Polymerization-Theory and Applications: Rapra Review, Vol. 9, Report, Rapra Technology (1998) and M. Tsunooka et al., Prog. Polym. Sci., 21, 1 (1996), it is an eagerly desired technique in the imaging field to obtain a photoinitiation system of high sensitivity. A photoinitiation system composed of a sensitizing dye and an initiator compound can generate an acid and a base in addition to the foregoing active radical by choosing the initiator compound and can be utilized in, for example, image forming such as photo shaping, holography, and color hard copying and in the field of manufacture of electronic materials such as photoresists and applied to photocurable resin materials such as inks, paints, and adhesives. In these industrial fields, in order to cause decomposition of an initiator compound with good efficiency, it is desired to find out sensitizing dyes having excellent light absorption and sensitizing ability.